In the past, people have felt that temperature-based control provided adequate comfort in closed spaces such as residential dwellings. The majority of such systems have an air recirculating system to heat the recirculated air if the space temperature as sensed by a thermostat is below a comfort range and cool the recirculating air if above the comfort range. It is well known that the relative humidity of air increases as its temperature is decreased, say by contact with a cool surface, until the dew point is reached, at which time relative humidity is 100% and water vapor starts to condense from the air onto the cool surface. In most cases the air conditioning unit is designed and its capacity is selected so that while cooling the air, air temperature within the air conditioner plenum adjacent to the heat exchanger is also lowered to below its dew point thereby condensing a portion of the humidity in the air passing through the plenum as well. When this cooled air with a part of its water vapor condensed in the air conditioner mixes with the air in the controlled space, the humidity of the space is reduced as well.
In many situations, this is adequate to assure comfort within the closed space. Experience shows that relative humidity need not be controlled as accurately as temperature in order to achieve comfort. Typically, if relative humidity can be held between approximately 30% and 50%, people within the closed space will be comfortable. In the majority of conditions, where high humidity is accompanied with relatively high temperature, the conventional approach outlined above is sufficient to keep relative humidity within or close to its comfort range. There are many situations however, where ignoring actual relative humidity achieved by conventional cooling is not adequate. For example, in some climates temperature is not far above the normal comfort range, but relative humidity is very high. Cooling the air results in a closed space with relative humidity far above the comfort range. In other situations, it is advantageous to control relative humidity more closely than the hit or miss approach of simply cooling recirculating air and thereby removing an indeterminate amount of water vapor from the air in the closed space. For example, glued furniture joints may weaken because of frequent humidity swings. Musical instruments such as pianos, harpsichords, guitars, violins, etc. may be damaged or their tuning harmed by frequent humidity changes. Certain house plants do not thrive if the humidity is consistently outside a relatively small range. Persistently high humidity can cause mildew of walls and clothes.
In larger environmental control systems it is possible to provide both cooled and dehumidified air from the air conditioning and heated air from the heating plant to the controlled space. In these systems, the heating and air conditioning can operate simultaneously. By providing more cooling than is needed to achieve the desired temperature in the controlled space, additional water vapor can be condensed from the recirculating air. Simultaneously providing heated air raises the space air temperature to within the comfort level.
Residential and small commercial spaces using recirculating air temperature control are not so easy to control in this way, and are the focus of this invention. The temperature in these spaces is controlled by a system including a plenum, a return air duct connected to provide air from the space to the plenum, and a conditioned air duct connected to allow air flow from the plenum to the space. A fan within the plenum creates a pressure differential within the plenum which extracts air from the space through the return air duct and forces the extracted air through the conditioned air duct into the space after its temperature and humidity has been changed. A heating unit which is usually either a furnace or heat pump and which operates responsive to a first value of a heating active signal has a heat exchanger within the plenum. An air conditioning unit operating responsive to a first value of a cooling active signal has a heat exchanger within the plenum downstream from the heating unit heat exchanger. A temperature sensor within the space supplies a temperature signal encoding a value indicative of the internal air temperature of the space and a humidity sensor within the space provides a humidity signal encoding a value indicative of the humidity level in the air within the space. A set point generator providing a temperature set point signal and a humidity set point signal respectively encoding a temperature set point value and a humidity set point value. Of course, the set point generator forms a part of the thermostat in most installations.
Because the furnace heat exchanger is typically positioned upstream in the duct or plenum from the air conditioning heat exchanger, it is obvious that providing heat to recirculating air just prior to it being cooled will not allow humidity control. Accordingly, a different procedure is needed to allow humidity control in spaces having such temperature control units.